I have a few of the old TSR boxed sets, most of which I bought at wild discounts when they were on end of print run clearance, because i was a penniless student at the time and if it wasn’t 80% off, then I couldn’t afford it. Dark Sun Revised, the FR grey box, Planescape, a couple of the Al-Qadim boxes (which were kinda half-sized and flimsy compared to the major boxed releases)
They had their ups and downs, but one thing I really miss was how they split the player-facing info about a setting from the DM secrets, adventures etc in different books. So convenient and such a great way of introducing a new player to a setting, simply being able to hand them a booklet and say ‘check this out’, rather than ‘check this out, but don’t read chapter 6 or 7, or the sidebar on page 29, or the last three paragraphs on page 16.